Osmolality Calculator
The Osmolality Calculator estimates serum osmolality based on sodium, glucose, and blood urea nitrogen (BUN) levels. This tool helps clinicians quickly assess fluid balance and electrolyte status in patients. Whether you are managing dehydration, detecting toxic alcohol ingestion, or evaluating hypernatremia, this tool provides instant, reliable results to support clinical decisions.
How Serum Osmolality Is Calculated
Serum osmolality measures the concentration of dissolved particles in your blood, reflecting how your body balances water and solutes. The calculation converts glucose and blood urea nitrogen to osmotically active units to ensure accuracy alongside sodium levels.
Osmolality = 2 × [Na+] + [Glucose]/18 + [BUN]/2.8 + [Alcohol]/4.6
Where:
- [Na+] = Sodium in mEq/L (doubled because it is the main extracellular cation)
- [Glucose] = Glucose in mg/dL (divided by 18 to convert to mmol/L)
- [BUN] = Blood urea nitrogen in mg/dL (divided by 2.8 to convert to mmol/L)
- [Alcohol] = Blood alcohol in mg/dL (divided by 4.6, if present)
This method is a standard calculation used to screen for significant imbalances. It provides a close estimate of the true measured osmolality, helping you identify potential gaps caused by unmeasured toxins like methanol.
What Your Serum Osmolality Means
This number tells you if your blood is too concentrated or too dilute, which is vital for proper cell and organ function. Understanding the result helps guide fluid therapy and identifies underlying metabolic conditions.
Normal Range (275–295 mOsm/kg)
A result within this range typically indicates healthy fluid balance. No immediate action is needed unless symptoms are present, but routine monitoring is advised for patients on IV fluids.
Hyperosmolality (>295 mOsm/kg)
Values above 295 suggest dehydration, high blood sugar, or kidney issues. Clinicians should investigate the underlying cause and may need to administer fluids or insulin to lower the concentration.
Hyposmolality (<275 mOsm/kg)
Low values can indicate overhydration or low sodium levels (hyponatremia). Careful restriction of fluid intake or medication to raise sodium levels might be necessary to prevent complications.
Osmolar Gap (>10 mOsm/kg)
If there is a large difference between this calculation and a lab measurement, unmeasured substances like alcohol or methanol may be present. This requires urgent testing for toxic alcohols.
Disclaimer: This calculator is for educational purposes only and is not a substitute for professional medical advice, diagnosis, or treatment. Always consult a qualified healthcare provider for specific medical concerns.
Re-evaluate the osmolality after starting treatment to ensure the patient is responding correctly to fluid management or medications.